1. Field of the Invention
The present invention relates to a liquid crystal display panel, and more particularly, to a transflective liquid crystal display panel and an apparatus and a method of driving the same capable of improving response time as well as providing a wide viewing angle.
2. Description of the Related Art
Generally, an active matrix liquid crystal display (LCD) device displays a natural moving picture by using a thin film transistor (hereinafter referred to as a TFT) as a switching device. Such an active matrix LCD device can be made smaller than a cathode ray tube CRT. Thus, active matrix LCD devices have been used as monitors for portable televisions and lap-top computers.
A liquid crystal display device is not self-luminous and thus, requires a separate light source. A liquid crystal display device may be classified as transmissive or reflective depending on the type of the light source. A transmissive liquid crystal display device includes two transparent substrates: an upper substrate and a lower substrate. A liquid crystal material is injected between the upper and the lower substrates. A backlight unit is positioned behind the lower substrate to project light onto a projection plane.
A reflective liquid crystal display device includes two transparent substrates: a rear substrate and a front substrate. A liquid crystal material is injected between the front and the rear surfaces. In contrast to the transmissive liquid crystal display device, the reflective liquid crystal display device has a specular surface formed on the rear substrate. The specular surface reflects an ambient light incident from the rear substrate through the front surface, which is the display surface. Alternately, the specular surface reflects a separate auxiliary light incident toward the display surface.
Recently, transflective liquid crystal display devices have been suggested that benefit from the advantages provided by both the transmissive type and the reflective type devices. In a transflective liquid crystal display device, if the level of externally applied ambient light is sufficient, the transflective liquid crystal display device does not require a back light unit. Instead, the transflective liquid crystal display device reflects the ambient light using a reflective plate. In this case, the transflective liquid crystal display device operates as a reflective liquid crystal display device. In contrast, if the level of externally applied ambient light is insufficient, the transflective liquid crystal display device uses a back light unit which provides light to a liquid crystal layer. In this case, the transflective liquid crystal display device operates as a transmissive liquid crystal display device.
FIG. 1 is a sectional view of a TN mode transflective liquid crystal display device according to related art. Referring to FIG. 1, the related art transflective liquid crystal display panel includes an upper plate UP and a lower plate DP formed with a liquid crystal material 18 therebetween, a reflective plate 6 formed in the lower plate DP, upper and lower retardation films 22 and 26, upper and lower polarizers 24 and 28 deposited on the exteriors of the upper plate UP and the lower plate DP, respectively, a scattering film 20 disposed between the upper retardation film 22 and an upper substrate 11, and a backlight unit 30 disposed on a rear surface of the lower polarizer 28.
The upper plate UP includes a black matrix 16, a color filter 12, a common electrode 14 and an upper alignment film (not shown) that are sequentially formed on the upper substrate 11. The black matrix 16 overlaps an area excluding a display area. The black matrix 16 prevents light leakage and absorbs ambient light, to thereby improve contrast. The color filter 12 is formed at an area partitioned by the black matrix 16 to selectively transmit light having a specific wavelength, thereby generating red R, green G and blue B lights. A common voltage is applied to the common electrode 14 to control movement of liquid crystal material.
The lower plate DP includes a TFT (not shown), a reflective plate 6, a pixel electrode 10 and a lower alignment film (not shown) formed on the lower substrate 1. The TFT is formed at a crossing of a gate line and a data line 4 and selectively supplies a data signal from the data line 4 to the pixel electrode 10 in response to a gate signal from the gate line. The pixel electrode 10 overlaps the reflective plate 6. A second passivation film 8 is positioned between the reflective plate 6 and the pixel electrode 10. The reflective plate 6 is made of aluminum within an area that overlaps a reflective portion RP on a first passivation film 2 to reflect ambient incident light.
The liquid crystal material 18 formed between the upper plate UP and the lower plate DP includes liquid crystal material of twisted nematic (TN) mode. The liquid crystal material of the TN mode has a twisted angle of 90° and transmits an incident light by changing its arrangement state when an electric field is applied thereto. The backlight unit 30 generates light required to display pictures when the liquid crystal panel operates in a transmissive mode.
Upper and a lower retardation films 22 and 26 are formed externally to the upper plate UP and the lower plate DP, respectively. The upper and lower retardation films compensate a phase difference, which causes a birefringence property. Specifically, a refractive index in a long-axis direction of the liquid crystal material differs from the refractive index in a short-axis direction of the liquid crystal material. The birefringence causes a difference in the polarization direction.
In the TN mode transflective liquid crystal display panel, the ambient light incident onto the reflective portion RP is reflected at the reflective plate 6 through the liquid crystal material 18 and is emitted back out of the liquid crystal display panel through the liquid crystal material 18. A visible ray generated at the backlight unit 30 and entering the transmissive portion TP propagates toward the liquid crystal material 18 through a transmissive hole TH to reach the display area. Specifically, a ray of light travels through the liquid crystal material 18 twice in the reflective portion RP. In contrast, a ray of light travels through the liquid crystal material 18 just for once in the transmissive portion TP.
Due to the foregoing property of the TN mode liquid crystal display panel, a cell gap in the reflective portion TP is made different than a cell gap in the transmissive portion TP to compensate for the optical phase difference. In other words, the reflective portion RP has a first cell gap d1 and the transmissive portion TP has a second cell gap d2. For example, the first cell gap d1 is about 3.5 μm.
The cell gaps d1 and d2 differ only by a depth of a through hole TH. Therefore, light transmittances of the transmissive portion TP and the reflective portion RP do not have a large difference, as known from following Equation 1.
                              T          =                      1            -                                                            sin                  2                                ⁢                                  π                  2                                ⁢                                                      1                    +                                          μ                      2                                                                                                                    1                  +                                      μ                    2                                                                                      ⁢                                  ⁢                  μ          =                                    2              ⁢              d              ⁢                                                          ⁢              Δ              ⁢                                                          ⁢              n                        λ                                              [                  Equation          ⁢                                          ⁢          1                ]            
In Equation 1, Δn represents a refractive index anisotropy, d represents a cell gap, which is the distance traveled by light through a liquid crystal layer, and λ represents a wavelength of the light.
FIG. 2 is a graph of light transmittance of the related art TN mode transflective liquid crystal display device depicted in FIG. 1. The light transmittances of the reflective portion RP and the transmissive portion TP randomly vary as shown in FIG. 2. Thus, it is difficult to optimize the brightness of the reflective portion RP and the transmissive portion TP. Further, the TN mode liquid crystal material 18 has a narrow viewing angle and a slow response speed.
FIG. 3 is a sectional view of an ECB mode transflective liquid crystal display device according to related art. A transflective liquid crystal display panel, which includes an electrically controlled birefringence (ECB) mode, has been proposed to overcome the problems associated with TN mode liquid crystal display panels. Referring to FIG. 3, the transflective liquid crystal display panel with the ECB mode includes an upper plate UP and a lower plate DP formed with a ECB liquid crystal material 19 therebetween. A reflective plate 6 is formed within the lower plate DP. Upper and lower retardation films 22 and 26 and upper and lower polarizers 24 and 28 are deposited on the exteriors of the upper plate UP and the lower plate DP, respectively. A scattering film 20 is positioned between the upper retardation film 22 and an upper substrate 11. A backlight unit 30 is positioned on a rear surface of the lower polarizer 28.
The upper plate UP includes a black matrix 16, a color filter 12, a common electrode 14 and an upper alignment film (not shown) that are sequentially formed on the upper substrate 11. The black matrix 16 is overlaps an area excluding a display area. The black matrix 16 prevents light leakage and absorbs ambient light, to thereby improve contrast. The color filter 12 is formed at an area partitioned by the black matrix 16 to selectively transmit light having a specific wavelength, thereby generating light of color red R, green G and blue B. A common voltage is applied to the common electrode 14 to control the movement of the liquid crystal material.
The lower plate DP includes a TFT (not shown), a reflective plate 6, a pixel electrode 10 and a lower alignment film (not shown) formed on the lower substrate 1. The TFT is formed at a crossing of a gate line and a data line 4 and selectively supplies a data signal from the data line 4 to the pixel electrode 10 in response to a gate signal from the gate line.
The reflective plate 6 covers a portion of an upper surface and a side surface of a first passivation film 2 around a first through hole TH1. The pixel electrode 10 overlaps the reflective plate 6. A second passivation film 8 is positioned between the reflective plate 6 and the pixel electrode 10. The reflective plate 6 is made of a reflective material at an area that overlaps the reflective portion RP to reflect an incident ambient light.
The pixel electrode 10 is formed on the second passivation film 8 and the second through hole TH2. The pixel electrode 10 overlaps the reflective plate 6 at an area partitioned by the data line 4 and the gate line. Further, the pixel electrode 10 is formed on the lateral side of the second passivation film 8 exposed by a second through hole TH2 on a substrate 1. The pixel electrode 10 is made of a transparent conductive material with a high light transmittance.
The reflective portion RP of the lower plate DP maintains a first cell gap d1 from the upper plate UP. The transmissive portion TP maintains a second cell gap d2 from the upper plate UP. The second cell gap d2 is twice the first cell gap d1.
FIG. 4 is a diagram illustrating a principle of operation of the related art ECB mode transflective liquid crystal display device depicted in FIG. 3. The liquid crystal material 19 provided between the upper plate UP and the lower plate DP includes liquid crystal material of an electrically controlled birefringence (ECB) mode in which liquid crystal cells are aligned parallel to alignment films (not shown) formed on the upper substrate 11 and the lower substrate and are moved in a direction parallel to the direction of an applied electric field. The backlight unit 30 generates light required for displaying pictures when the liquid crystal panel operates in transmissive mode.
Upper and lower retardation films 22 and 26 are formed on the exterior of the upper plate UP and the lower plate DP to compensate for a phase difference causing a birefringence property of the liquid crystal panel. Specifically, a refractive index in a long-axis direction of the liquid crystal material is different than a refractive index in a short-axis direction of the liquid crystal material causes. Birefringence causes a different direction of polarization.
Light transmittance of the transflective liquid crystal display panel of the ECB mode is expressed as Equation 2.
                    T        =                  1          -                                    1              2                        ⁢                                          sin                2                            ⁡                              (                                                      π                    ⁢                                                                                  ⁢                    Δ                    ⁢                                                                                  ⁢                    nd                                    λ                                )                                                                        [                  Equation          ⁢                                          ⁢          2                ]            
In Equation 2, Δn represents a refractive index anisotropy, d represents a cell gap, which is the distance traveled by light through a liquid crystal layer, and λ represents a wavelength of the light.
FIG. 5 is a graph representing the light transmittance of the transflective liquid crystal display device of the ECB mode shown in FIG. 3. The light transmittance T is periodically repeated by a cell gap d as expressed by Equation 2. Specifically, the light transmittance of the reflective portion RP having the first cell gap is twice the light transmittance of the transmissive part TP having the second cell gap d2. The second cell gap d2 is twice the first cell gap d1 as shown in FIG. 5.
Accordingly, if the second cell gap d2 is formed larger than the first cell gap d1, the light transmittance T of the reflective portion RP becomes identical to the light transmittance T of the transmissive portion TP. Thus, it is possible to optimize the brightness of the reflective portion RP and the transmissive portion TP.
However, since the liquid crystal display panel of the ECB mode has a response speed of several tens of milliseconds and a narrow viewing angle, it is difficult to use the ECB mode liquid crystal display panel when a wide viewing angle is required. Further, since the response speed of the liquid crystal material is inversely proportional to the square of the cell gap, the response speeds of the transmissive portion and the reflective portion differ significantly. Accordingly, when displaying moving pictures, which require higher response time than still pictures, a quality of the displayed pictures through the transmissive portion and the reflective portion of the display device deteriorates.
In addition, in the liquid crystal display panel of the ECB mode, the liquid crystal material cannot be aligned uniformly in the inclined region A around the second through hole TH2. Thus, the liquid crystal display suffers from a disclination phenomenon, which causes a deterioration in picture quality.